Conventionally, the surface of a moving web of paper or board is smoothed and made glossy in a multiroll calender comprising a plurality of rolls stacked in a calender frame so as to form a nip contact with each other. A multiroll calender comprises a top roll and a bottom roll with at least one intermediate roll located therebetween. The rolls of the stack are compressed against each other by the top and bottom rolls that act as the loading rolls to provide a sufficiently high linear nip force. In calendering, the web passes through the calender nips formed by the superimposed rolls. A soft-roll calender typically has two or four nips placed in a succession in separate calender frame sections thus allowing the web being calendered to pass straight between two successive nips. In a conventional two-nip soft-roll calender, the web is calendered once on either side.
Some of the calender rolls are heatable, hard-surfaced thermorolls, while some others are rolls surfaced with a soft coating. For instance, one of the rolls in a roll pair of a soft-roll calender is generally a metal-surfaced thermoroll and the other is soft-coated roll. The thermoroll is typically heated with oil or some other heat-transfer medium such as water or steam. Using oil as the heating medium, a roll temperature of almost 300° C. can be reached. However, the temperature of the thermoroll is typically about 200° C. Circulation of the heat-transfer medium in the roll can be accomplished in different ways. The most frequently employed technique is to feed the heat-transfer medium into and out from the roll via a single end only, whereby the medium first is passed along one duct to the roll end and then the return flow takes place along a parallel duct. The return flow exits the roll to a reheating circuit via a bore made to the roll end flange and the roll shaft.
In the material selection for soft-coated rolls, the possible temperature elevation caused by an adjacent thermoroll must be taken into account. The surface coverings of soft-coated rolls are selected from the group of materials compatible with the rugged conditions imposed by the nip contact, such as a generally available polymer of the thermosetting or thermoplastic type, for instance. Rolls coated with a thermoplastic polymer are described in, e.g., publication GB 1,011,114, while rolls coated with a thermosetting polymer are described in, e.g., publication EP 321,561. In the prior art it was conventional to make the covering of soft-coated rolls from natural-fiber-based materials such as those described in, e.g., publication U.S. Pat. No. 4,283,821.
As the web being calendered may in some cases break during calendering, either prior to or after the calendering step, it is necessary to have an ability to open the calender nips as rapidly as possible at the occurrence of a web break. Opening the nips is particularly important in calenders having soft-coated rolls. Namely, the thermorolls of closed nips begin after the web break to heat the adjacent soft-coated rolls because there is no more paper web running through the nip so as to remove the heat emitted by the thermorolls. Consequently, the soft-coated rolls may overheat resulting in a roll becoming damaged. In modern calenders running at high web speeds, also the amount of heat transferred to the thermorolls may be substantially large, which means that an extremely rapid opening of a thermoroll nip is mandatory, even as fast as a few tenths of a second if the web break occurs just upstream from a nip.
To prevent damage to soft-coated rolls, calenders are equipped with automatic web-breakage control that opens the calender nips immediately at a web break. Typically, the occurrence of web breaks is monitored by means of photocell curtains installed over the web in the cross-machine direction. As the detection of a web break must take place in a short time, it is not possible to apply a sufficient filtration to the output signal obtained from the photocells. Hence, a small defect, such as a hole in the web, may be interpreted as a web break even if the defective portion of the web in fact might pass the calender without problems. However, due to the high price of the soft-coated rolls, it is customary to set the control system to open the nips already at the occurrence of the smallest defects. As a consequence of such an erroneous or oversensitive response of the automatic web-breakage control system, the calender nips are opened, which is an ultimate cause to an actual web break.